In one type of radar system, the transmitter is driven by a CW signal having a frequency that varies linearly with time. The signal is derived from a voltage controlled oscillator (VCO), and the input signal to the VCO is controlled to produce the linear frequency sweep. The return signal from a target is mixed with the transmitted signal to produce a difference signal having a frequency equal to the difference between the instantaneous frequency of the transmitted signal and the return signal. Because the frequency of the transmitted signal changes linearly with time, the frequency of the difference signal is a function of target range.
In the system of the type described above, there are three fundamental problems that must be solved to develop an effective radar. The first problem is that the frequency sweep must be highly linear, in order for the radar to have adequate range resolution. This is a difficult problem, because even the best RF sources have nonlinear tuning characteristics, i.e., the frequency of the VCO output signal is not a linear function of the input voltage. The second problem relates to radar systems in which the antenna is scanned. In such systems, the sweep rate must be fast enough to allow the required scanning rates. The requirement for a fast sweep rate is a particular problem when a real time closed loop linearization scheme is to be used. This problem results from the fact that faster sweep rates require wider bandwidth loops, but loop bandwidth can be expanded only to the point where stability is threatened by component limitations such as amplifier bandwidths, delay, and unwanted capacitance. The third problem in designing an effective FM CW radar is that the linearization of the sweep must be able to adapt immediately to changes in the tuning characteristics of the VCO, which tuning characteristics depend on the terminating impedance at the RF output. This terminating impedance does not remain constant because the antenna must scan, and because polarization switching may occur.
Past efforts to product a fast, linear frequency sweep have involved both open loop and closed loop designs. To date, open loop techniques have proved nearly impossible to align. If and when they are aligned, they are still sensitive to changes in temperature. Closed loop designs have included phase lock loops in which a signal representing the rate of change of the frequency of the transmitted signal is phase locked to a crystal source. Such techniques in the past have been unable to follow a frequency slope with zero error. Prior phase lock loop attempts have also required a summation amplifier to drive the tuning input of the RF source. However, it has been found that it is very difficult to design such a circuit with a sweep rate fast enough to allow the use of a scanning antenna and with high range resolution.